(1) Field of the Invention
This invention relates to a semiconductor device and a method for fabricating the semiconductor device and, more particularly, to a semiconductor device having metal wirings of copper (Cu) or the like and a method for fabricating such a semiconductor device.
(2) Description of the Related Art
With an increase in the integration levels of silicon (Si) semiconductor devices and a reduction in the sizes of chips, wirings formed in these semiconductor devices have become thinner and the number of wiring layers formed in these semiconductor devices has increased. With devices in which a half pitch is 65 nm, for example, the minimum wiring width is about 100 nm. When an electric current is passed through such a thin wiring, metal atoms contained in the wiring may move. This phenomenon is referred to as electromigration. The electromigration causes a void or a hillock in the wiring, resulting in an increase in the resistance of the wiring, the breaking of the wiring, a short circuit, or the like. That is to say, the electromigration deteriorates the reliability of a circuit.
By the way, copper wirings formed by what is called a damascene method are generally used in ultramodern Si semiconductor devices. With the damascene method a copper wiring is formed in the following way. For example, lithography and etching are used first for forming a groove in an insulating film. A barrier metal and a seed copper film are then formed over an entire surface by a sputtering method. A copper film is formed over the barrier metal and the seed copper film by an electroplating method to fill in the groove. An excess copper film and barrier metal over the insulating film are removed by chemical mechanical polishing (CMP). By doing so, a copper wiring is formed. In addition, a cap film of silicon nitride (SiN) or the like is formed on the copper wiring by, for example, a chemical vapor deposition (CVD) method for the purpose of suppressing the diffusion of copper atoms contained in the copper wiring. This is the same with the above barrier metal (see, for example, Japanese Unexamined Patent Publication No. 2005-317835).
The copper wiring formed in this way is enclosed by the barrier metal and the cap film. Adhesion at an interface between the cap film and the copper wiring is poorer than adhesion at an interface between the barrier metal and the copper wiring. The reason for this is as follows. The two metals are joined at the interface between the barrier metal and the copper wiring. On the other hand, the insulating film and the metal are joined at the interface between the cap film and the copper wiring. It is considered that diffusion of copper atoms contained in the copper wiring tends to occur at the interface between the cap film and the copper wiring where adhesion is poor. Really the probability that voids are found here and there at this interface is high.
For example, the following method is proposed as a method for increasing resistance to the electromigration. Before a cap film is formed, predetermined pretreatment is performed. By forming the cap film after the pretreatment, the properties of an interface between the cap film and a copper wiring are changed (see, for example, “Identification of Electromigration Dominant Diffusion Path for Cu Damascene Interconnects and Effect of Plasma Treatment and Barrier Dielectrics on Electromigration Performance”, Proceedings of the 42nd International Reliability Physics Symposium (IEEE, Phoenix, U.S.A. 2004), pp. 246-250). In addition, the following method is proposed as another method for increasing resistance to the electromigration. A metal such as cobalt tungsten phosphorus (CoWP) is used for forming a cap film (see, for example, Journal of Applied Physics, Vol. 93, No. 3, pp. 1417-1421, (January 2003)).
As generations progress in the future, wiring width becomes narrower. With devices in which a half pitch is 45 nm, the minimum wiring width is about 70 nm. In such a situation, it may be impossible to adequately suppress the electromigration only by taking a material for the cap film into consideration and changing the properties of the interface between the cap film and the copper wiring in the above way. For example, if SiN is used for forming the cap film, adhesion at the interface between the cap film and the copper wiring is good compared with the case where silicon carbide (SiC) or silicon carbide nitride (SiCN) is used forming the cap film. However, the dielectric constant of SiN is higher than that of SiC or SiCN. As a result, forming the cap film by the use of SiN makes high-speed operation of the device impossible.
In addition, if a metal cap film like that described above is used, adhesion at the interface between the cap film and the copper wiring is good and the diffusion of copper atoms can be suppressed. Such a metal cap film must selectively be formed over wirings. However, it is not always easy to selectively form a metal cap film in that way over narrow-pitch wirings in a next generation semiconductor device. Accordingly, under the present situation a problem still exists from the viewpoint of mass production.